If you want to get both nulls and an assoc array, you have to ADD the values like this:
$row = oci_fetch_array($stmt, OCI_RETURN_NULLS + OCI_ASSOC);
This really should be noted in the text of the manual.

As Robert Hicks mentioned back in August 2004 there is an error in examples 3 and 4 of this page. The error in example 3 is what dwhitaker and stry_cat address in their notes of 20 May 2005 and 9 June 2005 respectively.
The correct form of example 4 should read:
<?php
$connection = oci_connect("user", "password");
$query = "SELECT id, name, lob_field FROM fruits";
$statement = oci_parse ($connection, $query);
oci_execute ($statement);
while ($row = oci_fetch_array ($statement, OCI_RETURN_LOBS)) {
echo $row[0]."";
echo $row[1]."";
echo $row['LOB_FIELD'].""; //this will output LOB's content
}
?>
This really should be corrected in the actual documentation...